We demonstrate that a single Gaussian-like beam can self-bend during nonlinear propagation in a uniform photonic lattice. The two components of the beam experiencing normal and anomalous diffractions spontaneously separate and form a pair in a diametric-drive acceleration due to nonlinear action. Such a diametric drive generally describes a jointly accelerating behavior of two beams analogous to positive- and negative-mass objects. The influences of the initial momentum of the input beam and the nonlinear strength are considered in this process. We further realize a self-bending propagation for a partially coherent light beam and discuss the impact of incoherence on the acceleration strength.